OBJECTIVES This study investigated post-traumatic stress disorder (PTSD) symptoms and illness perceptions in people who suffered the acute medical trauma of a myocardial infarction (MI) or a subarachnoid haemorrhage (SAH). The study tested hypotheses regarding changes in PTSD symptoms and illness perceptions over time, associations between PTSD and illness perceptions and cognitive predictors of PTSD. DESIGN AND METHOD The study employed a longitudinal design and measured the illness perceptions and PTSD symptoms of an MI group (N=17) and a SAH group (N=27). Data were collected within 2 weeks of admission (T1), 6 weeks after admission (T2) and 3 months after admission (T3). Statistical analysis was undertaken to examine associations between illness perceptions and PTSD and to examine cognitive predictors of PTSD. RESULTS The prevalence of PTSD within the total acute medical trauma sample was 16% at 2 weeks, 35% at 6 weeks and 16% at 3 months. Illness perception factors of identity, timeline (acute/chronic), consequences and emotional representation were strongly correlated with PTSD at all three time points. PTSD symptoms and illness perceptions were shown to have changed over time. The results also showed that several illness perception factors are significant predictors of PTSD. CONCLUSIONS Both PTSD symptoms and illness perceptions changed significantly over time following an MI or SAH. Illness perception factors are significant predictors of PTSD.